Diagnosis: Transient Synovitis (Transient Tenosynovitis)
The most likely diagnosis is transient synovitis (option C), given the 7-day duration of hip pain and limping with preserved range of motion and no limb length discrepancy, though serious pathology including slipped capital femoral epiphysis must be excluded urgently.
Clinical Reasoning
Why Transient Synovitis is Most Likely
Preserved range of motion is the key distinguishing feature in this case—transient synovitis typically presents with pain but maintains hip mobility, whereas other serious conditions cause significant movement restriction 1.
The 7-day duration fits the typical course of transient synovitis, which is self-limited and usually resolves within 1-2 weeks 1.
No shortening effectively rules out significant hip dislocation or advanced slipped capital femoral epiphysis, which would cause limb length discrepancy 1.
Critical Conditions That Must Be Excluded
Slipped Capital Femoral Epiphysis (SCFE)
This is the most commonly missed diagnosis in children and adolescents presenting with hip pain and must be actively excluded 2, 3, 4.
SCFE typically occurs in children aged 8-15 years, particularly those who are overweight or experiencing growth spurts 2, 5.
Classic presentation includes: limping, poorly localized pain (hip, groin, thigh, or knee), obligate external rotation with hip flexion, and decreased internal rotation 2, 3, 6.
Knee pain is a common presenting complaint due to referred pain from the hip—every child with knee pain must undergo hip examination 3, 4.
The absence of restricted movement in this case makes SCFE less likely, but it cannot be excluded on clinical grounds alone 2, 3.
Delayed diagnosis allows stable SCFE to become unstable, dramatically increasing the risk of avascular necrosis and lifelong disability 3, 4, 5.
Septic Arthritis
If there are any signs of infection (fever, elevated inflammatory markers, refusal to bear weight), septic arthritis must be considered the diagnosis until proven otherwise 1.
Septic arthritis typically causes severe pain with any movement and significant restriction of range of motion, which is not present in this case 1.
Immediate Diagnostic Approach
Mandatory Imaging
Bilateral hip radiographs (anteroposterior and frog-leg views) must be obtained immediately to exclude SCFE and other serious pathology 2, 3, 4.
Radiographs are essential even when clinical suspicion is low, as SCFE findings can be subtle and easily missed 4.
Both hips must be imaged because SCFE is bilateral in 20-40% of cases 2.
Hip Ultrasound
If radiographs are normal and symptoms persist, hip ultrasound should be performed to detect joint effusion, which is present in both transient synovitis and septic arthritis 1.
Ultrasound can guide aspiration if infection cannot be excluded clinically 1.
Why Other Options Are Incorrect
Short Femur (Option B)
Congenital limb length discrepancy would be present from birth, not presenting acutely with a 7-day history 1.
The examination specifically notes no shortening, eliminating this diagnosis 1.
Muscle Tear (Option D)
Muscle tears cause localized tenderness and pain with specific movements, not generalized hip pain 1.
A 7-day duration without improvement would be unusual for a simple muscle strain in a child.
The clinical presentation of limping with hip pain in a child warrants investigation for hip joint pathology, not soft tissue injury 1.
Critical Pitfalls to Avoid
Never dismiss hip pathology based on knee pain alone—referred pain from the hip commonly presents as knee pain in children 3, 4, 6.
Never delay imaging in a limping child—SCFE is frequently missed at initial presentation due to failure to obtain hip radiographs 3, 4, 5.
"Groin pull" is exceedingly rare in children and should be a diagnosis of exclusion only after hip pathology is ruled out 3.
Preserved range of motion does not exclude serious pathology—early SCFE may have minimal restriction 2, 3.